NASA's Inspiring, Enlightening, and Successful Search for New Earths

The Kepler space telescope, NASA’s first mission dedicated to the search for planets beyond our solar system, has produced a gusher of strange new worlds. If astronomers are right, many of them will prove to be habitable.

﻿A journey that began four decades before Columbus sailed for the New World finally ended when the Kepler space telescope snared a few errant photons as they shot past Earth’s orbit en route to infinity. The light had sped through space for 560 years, traveling more than three quadrillion miles from a star much like our sun. Captured by Kepler’s digital sensors, transformed into bytes of data, and downloaded to computers at NASA’s Ames Research Center near San Francisco, the processed starlight slowly revealed a remarkable story: A planet not much bigger than Earth was whipping around its native star at a blistering pace, completing an orbit—its version of a “year”—in just over 20 hours.

Aside from its size, the planet bears little resemblance to Earth. It circles so close to its star that its surface temperature probably exceeds 2,500 degrees Fahrenheit, hot enough to melt iron. Nevertheless, the planet’s detection was a technical and intellectual coup, a rite of passage for Kepler. The planet, dubbed Kepler-10 b when NASA announced its existence this past January, was the smallest world yet found beyond our solar system. Its discovery proved that the Kepler spacecraft, which was launched in March 2009, could indeed do what its designers had boldly promised: find small, Earth-size planets around distant stars, a task that once seemed so difficult as to border on the absurd.

Kepler-10 b was merely a preview. A month after the January announcement, NASA released its first full data set from the Kepler mission, and the results left astronomers straining for superlatives. “Frankly, we’re overwhelmed,” says Geoff Marcy, an astronomer at the University of California, Berkeley, and a member of the Kepler team. “What NASA is doing is akin to the transoceanic voyages of the 15th century—the voyages that opened up the whole world. With the Kepler telescope, we’re learning about the properties of planets across the cosmic ocean. This is history. It’s Armstrong stepping off the bottom rung.”

Kepler monitors 156,000 stars, less than 0.0001 percent of the galaxy’s population. Like some cosmic wildcat drilling operation, Kepler has struck a gusher, discovering 1,235 possible new worlds in its first four months of operation. That number doubles the previous total of just over 500, painstakingly gathered over the last 16 years. Prior to 1992, keeping track of all the known planets around other stars like the sun was easy—the tally stood at an even zero.

The bulk of Kepler’s data have not yet been studied, and the mission will keep going for at least two and a half more years. But it is already shredding the textbooks, showing that our galaxy (at least the fraction of it seen by the spacecraft) contains a far more exotic assortment of planets than astronomers expected to find. “We’re learning about a diversity of worlds in our universe that we had no clue about beforehand,” Marcy says. “Rocky planets, yeah, we thought there might be some of those. By the way, we’re finding some rocky planets that are even denser than Earth. But we’re also finding these mini-Neptunes, a class of planet for which we have no examples in our solar system. They’re like small Neptunes but with huge amounts of liquid water around a rocky core.”

Also on the list are 67 planets roughly the size of Earth, give or take a thousand miles or so in radius﻿; 288 “super-Earths” up to twice Earth’s diameter; 662 Neptune-size planets; and 184 giants rivaling or exceeding Jupiter in size. The simple statistics from Kepler say that Earth-size planets are widespread. But as William Borucki, the head of NASA’s Kepler team, points out, Earth-size does not mean Earth-like. Many of these are inhospitable, roasting-hot worlds.

The most intriguing discovery from Kepler is that 53 of those 1,200-odd planets dwell in the life-friendly “Goldi­locks” zones of their stars, regions where temperatures would be just right—not too cold and not too hot—for liquid water. Where there’s water, scientists reason, there could be life. Four of these Goldilocks planets are super-Earth size or smaller, meaning that they could actually resemble our own home. Even the giant worlds may offer hope. “Some of the Jupiter-size planets could conceivably have Earth-size moons, and those moons would of course also be in the habitable zone,” Borucki says.

Just two years into its mission, Kepler is well on its way toward determining whether planets like Earth are rare or common. But that is just the first domino as scientists try to topple the much bigger questions, the kinds that make the hairs stand up on the back of your neck. Does life exist on other planets? Are planets with life common? Are any other intelligent beings out there?

For the first time, we have a handle on the odds, and the numbers beaming in from Kepler are not only encouraging but staggering. “Our galaxy contains 200 billion stars,” Marcy says. “I would guess that at least 30 percent of them have an Earth-size planet. So 30 percent of 200 billion, that’s at least 60 billion Earth-size planets just in our galaxy alone.”

When astronomers started finding planets around other stars in the 1990s, they fully expected to see the general structure of our own solar system repeated throughout the cosmos. Small, rocky planets like Mercury, Venus, Earth, and Mars would orbit close to the star. Giant, gaseous planets would occupy more distant orbits, just as Jupiter and Saturn do in our solar system.

The reasoning seemed self-evident. The heat from a parent star would tend to blast away lightweight elements like helium and hydrogen. Only rocky, sturdy planets could form nearby; giant planets would form farther out, where ices and cool gases could gather together. Astronomers had no way to check their assumptions, though, because for most of the 20th century there were no telescopes capable of detecting planets around other stars. The few astronomers who even attempted to look for them languished in obscurity, spending years in fruitless searching. Such was the state of astronomy when Borucki began his career at NASA.

Borucki, 72, is a mild-mannered man, patient with bureaucrats and unruffled by skeptics, traits that served him well in his three-decade quest to find those alien worlds. Without him there would be no Kepler spacecraft, and this year’s bumper harvest of planets would not have happened.

Borucki joined NASA’s Ames Research Center in 1962, straight out of the University of Wisconsin with a master’s degree in physics. He couldn’t imagine a better job. As a boy growing up in rural Wisconsin, he had launched homemade rockets. His first assignment at NASA—researching heat shields for the Apollo moon missions—suited him perfectly. Like everyone else who worked at the space agency in those heady days, Borucki dreamed of exploring other worlds. NASA Ames was also the headquarters for SETI, the Search for Extraterrestrial Intelligence, and Borucki became friendly with one of SETI’s founders, the visionary astronomer Carl Sagan.

After the Apollo program ended, Borucki grew fascinated with the idea that Earth-like planets might be orbiting other stars, unseen. He was particularly intrigued by a presentation on transit photometry, a theoretical technique that would make it possible to bring those planets into view. The concept was to equip telescopes with extremely sensitive electronic light detectors that could record the slight dip in brightness that occurs when a planet passes in front of a star. It was a brilliant idea, but it was a case of theory outpacing technology; existing detectors were not precise enough to measure such small variations in starlight consistently.

That same year Borucki even managed to get a bit of funding from NASA to host a conference dedicated to transit photometry. During the conference, some scientists from the National Bureau of Standards suggested that silicon diode detectors might provide the kind of precision that Borucki needed for transit photometry. When struck by a particle of light, the detectors emit a single electron. And converting starlight efficiently into a detectable electronic signal was crucial for transit photometry.

The problem was that each detector could track just one star, and Borucki knew that stringing thousands of detectors together to track enough stars to make the statistics meaningful was wildly impractical. “Every year I’d go to my branch chief for a performance evaluation. Every year he would tell me, ‘Bill, this is never going to work.’ And I never got promoted. There wasn’t much support. On the other hand, no one ever tried to stop me. NASA Ames has a director’s discretionary fund, and anyone who has a new idea can apply for it. That supported me for quite a while.”

By 1992 Borucki’s efforts had advanced to the point where he had assembled a team that began making formal proposals for a fully funded mission. He had determined that chips called charge-coupled devices, or CCDs (the kind used in many digital cameras), could provide the precision needed to accurately measure extremely small changes in starlight. Equally important, they could be packed into arrays capable of monitoring thousands of stars simultaneously. Borucki’s team submitted a plan for a mission it called FRESIP, short for Frequency of Earth-Sized Inner Planets. NASA rejected it, citing lack of proof that the CCD technology would work.

Borucki persisted. When NASA announced that it was accepting proposals for the Discovery program, a new class of low-cost, three-year space missions, he immediately applied. NASA’s reviewers estimated that his proposed mission would be too expensive. Borucki disagreed, of course, and in 1996 he tried yet again.

This time his project no longer seemed so implausible. The year before, a Swiss team had found 51 Pegasi b, a remarkable planet beyond our own solar system—the first ever discovered around another sunlike star. It is a gas giant like Jupiter, but it orbits just 5 million miles from its sun, racing around the star in only four days. (For comparison, Jupiter orbits at a distance of 480 million miles and takes nearly 12 years to complete one circuit.) The finding completely overturned the old assumption that all planetary systems would look like ours and suggested that astronomers had been overlooking a whole class of planet. “All of a sudden, it went from ‘Anybody who is doing it is crazy’ to ‘Anybody who’s doing it is at the forefront of astronomy,’ ” Borucki recalls.

The Swiss team made its discovery using a ground-based technique pioneered by Geoff Marcy and Canadian astronomer Bruce Campbell in the 1980s. Known as the Doppler method, it measures the gravitational tug exerted on a star by a planet—a planet that could not be seen directly because it would be lost in the glare of its star. Astronomers could use Doppler measurements to find large, Jupiter-size planets, but to find smaller worlds they would need something more precise, something like the mission Borucki had in mind. “The technique has a limited ability,” Marcy says. “It’s not going to take us all the way to Earths that orbit as far from their stars as our own Earth orbits the sun.”

Borucki, meanwhile, had also attracted some heavyweights to his cause. Carl Sagan had joined the team, and shortly before his death in 1996 he supported a suggestion to change the mission’s name to something catchier than FRESIP. David Koch, a physicist at Ames who had been working on the mission with Borucki since 1991, suggested Kepler, after the great 17th-century astronomer who discovered the laws governing the orbits of planets around the sun. Kepler it was.

Borucki’s mission plan now called for a spacecraft that would trail behind Earth as it orbited the sun. Being so far from Earth would give the telescope an unobstructed view of its target stars. It would be equipped with a 56-inch telescope to focus faint starlight on an array of 42 CCDs. By comparison, the cutting-edge Hubble Space Telescope, launched six years earlier, had just four detectors. Borucki felt confident. But NASA rejected the proposal yet again, questioning whether the hardware and software could simultaneously track thousands of stars.

Borucki’s team went back to the lab again, this time building a miniature observatory: a 2-foot-long, 8-inch-wide telescope with a CCD array. Coupled with software to reduce assorted stellar background noise, it could measure light changes down to 20 parts per million, making it more than sensitive enough to detect an Earth-size planet around a sunlike star in an orbit as large as Earth’s. With that, another proposal followed in 1998, but it, too, was rejected. By now, though, NASA’s objections were becoming pickier: Could the telescope withstand the rigors of launch and perform reliably once in orbit?

To satisfy all the remaining objections to the project, the team organized a demonstration that proved Kepler could control its vibrations and temperature enough to withstand the rigors of space. Borucki submitted yet another proposal in 2000. At long last, NASA accepted it.

“After struggling for all those years,” Borucki says, “it was exhilarating to finally hear a NASA review panel say not only that it could be done, but that it should be done.”

Three planets in a six-planet solar system discovered in February simultaneously transit their parent star, Kepler-11.

NSA/TIMPYLE

The next year, NASA provided the Kepler team with a budget of $299 million and aimed for a launch date sometime in 2006. The job of transforming the vision of Borucki’s team into a functioning spacecraft fell to Ball Aerospace in Boulder, Colorado. John Troeltzsch, the Kepler program manager at Ball, says the task was far from straightforward. “When a science team proposes to NASA to build a mission like this, the team crafts a story about how few technical advances are required,” he says. “They’ll say ‘We’ve got the detectors; we’ve got the optics.’ They make it sound easy. Well, then you’ve got to really build it, and all the things you thought would be easy aren’t. One example is the CCD detectors. People have been flying CCDs in space now since the mid-90s. So the idea of taking a whole bunch of CCDs and putting them together—how hard could that be?”

It turned out to be hard enough to keep 2,000 engineers and technicians at Ball working more than a million hours over five years. The end result was a 2,320-pound spacecraft equipped with an outsize digital camera—made of 22,000 parts—and a 55-inch telescope, the largest NASA has ever launched beyond Earth orbit. Kepler ended up costing $600 million. The CCD array measures 11 inches by 11 inches, and the whole camera sits in a 1.5-cubic-foot box suspended in a graphite-epoxy frame in the middle of the telescope. “The detector is basically a 95-megapixel camera,” Troeltzsch says. Besides being hardened against radiation, the CCDs must be kept very cold, at –120ºF, to detect very low levels of light. Meanwhile, the electronics controlling the camera, positioned just inches from the CCDs, run about 225 degrees warmer. Aluminum pipes passively conduct heat from the electronics to a radiator mounted on Kepler’s surface.

After some funding delays, NASA finally launched Kepler on March 6, 2009, 17 years after Borucki’s first formal proposal and 400 years after Johannes Kepler published his first two laws of planetary motion in his book Astronomia Nova. A Delta II rocket sent Kepler into a sun-centered orbit similar to the one that Earth follows, in perfect clockwork accord with Kepler’s laws.

The launch, Troeltzsch says, was nerve-racking. “You build this huge, high-fidelity instrument, and then you put it on a rocket that is 20 stories high and you explode it into space. It takes about two days to settle down and say OK, we’re good, and about 30 days before you really start to relax.”

“Kepler is the most boring mission you can imagine,” Jon Jenkins tells me, smiling. “It simply stares at one place in the sky.” That simplicity has turned out to be a significant factor in Kepler’s success; it greatly reduces the number of things that can go wrong. “We never change Kepler’s attitude except to rotate it once every 90 days to keep its solar arrays pointed at the sun, and the sun shades in place to keep the sunlight out of the barrel of the telescope. We’re just staring and waiting for the stars to wink at us. You never know when that’s going to happen—when a planet is going to cross in front of a star.”

Jenkins, an electrical engineer by training, has been on the Kepler team since 1995; his office is just down the hall from Borucki’s. The Kepler mission, his own assertion notwithstanding, has never bored him. Jenkins spent years designing and refining the sophisticated software that sifts the raw data downloaded from Kepler, looking for the telltale changes in stellar brightness that might reveal the existence of a new world. “Turning pixels into planets” is how he describes his work.

For its entire mission, Kepler’s view will remain fixed on one of the spiral arms of our galaxy, on a field of stars in the constellations Cygnus and Lyra. Most of the stars it sees lie between 600 and 3,000 light-years from Earth, but some are as close 30 light-years. The cyclic dimming of more distant stars would be too faint for Kepler to measure. Although Kepler cannot peer nearly as far into the cosmos as the Hubble telescope, its view is far more panoramic, as wide as 27 full moons across the sky. Hubble focuses on a dot of sky no larger than what you would see by looking through a coffee stirrer.

The challenge for Kepler—or more specifically, for Jenkins’s software—is to tease out brightness changes caused by the passage of a planet and to distinguish them from all the normal stellar variations, such as flares and star spots (the stellar equivalent of sunspots) or even nearby eclipsing stars. As with many things in life, timing is crucial. Planets give themselves away by the length of time it takes them to pass across the face of a star—typically a few hours. Star spots, which are embedded in a star’s surface, typically rotate on a scale of days or weeks. So Jenkins’s software searches for dips in brightness lasting up to half a day. If a planet is indeed the cause of the change in brightness, the exact same change should recur days, months, or years later, depending on how long the planet takes to orbit its star. Ideally, the Kepler team waits until the spacecraft has recorded three identical dips in brightness separated by equal intervals before concluding that they have probably found a planet.

Kepler’s onboard computers can store a bit more than two months of data; the data are highly compressed for efficient storage and transmission to Earth, where NASA’s servers can hold a total of about 60 terabytes. Without the compression scheme, the Kepler mission never would have flown. “It would have taken us five times as long to downlink the data and five times as much hardware to store the data on board,” Jenkins says. “Compression was absolutely crucial to the mission.” As it is, the spacecraft has already beamed down more than a terabyte of brightness measurements. Kepler astronomers have enough data to keep them busy for decades.

NASA is planning to release results from Kepler regularly through November 2013, when the mission is scheduled to end. The most sought-after results still lie ahead. After Jenkins and his colleagues have weeded out sunspots and other planet poseurs from the data, Marcy and other astronomers use the Doppler wobble method with terrestrial telescopes to verify that the remaining planet candidates, or “objects of interest,” are indeed planets. The process will demand at least three years to find a completely Earth-like planet: one that is in a yearlong, Earth-like orbit around a star just like the sun.

The grand prize for Kepler would be to find a world just like ours—no bigger, no smaller—orbiting a sunlike star in an orbit the same size as Earth’s. That challenge lies at the very edge of Kepler’s remarkable capabilities. And it is definitely beyond the reach of even the most powerful Earth-bound telescopes.

Doppler measurements have been used to confirm the existence of Earth-size planets discovered by Kepler in habitable zones around stars smaller and dimmer than the sun. Kepler-10 b is one such example. But Doppler measurements won’t be able to confirm a true twin of Earth: A planet’s gravitational effect on its star depends on their relative mass and the distance between them. An Earth-size world close to a lightweight, dim star is much easier to find than a planet more like our own. Marcy laments, “We won’t ever definitely verify or measure the density of true Earth clones that orbit as far as our own Earth does from the sun.”

There is a chance, though, that Kepler will get lucky. If the spacecraft finds an Earth-like planet circling a star orbited by at least one other planet, Kepler’s data alone could be used to determine the masses of the planets without measuring the star’s wobble. The gravitational interactions of the two planets would affect the timing of their orbits, which is something Kepler could measure. And the gravitational interactions, in turn, would reveal the planets’ masses. The Kepler team has already pulled off that feat for two planets orbiting a star called Kepler-9, about 2,000 light-years from Earth. The planets orbiting Kepler-9, astronomers determined, are such low density they would float if placed in a giant bathtub filled with water. Nobody understands the nature of these bloated worlds. “It’s a beautiful, beautiful example of what Kepler can do,” says Natalie Batalha, the mission’s deputy science team leader.

Kepler may well find planets that closely resemble Earth. Or it may continue to upend our notions of what is out there. “All planetary systems were supposed to look like ours,” Borucki reminds me. “The planets we’re finding are in the wrong places! And their orbits are unlike anything anyone predicted. Now, that’s a warning. That tells you we don’t know how to predict what’s out there.”

Kepler has proved that planets are the rule in the galaxy, not the exception, but it won’t tell us if any of those planets actually harbor life. That’s a question for future missions, and NASA has plans for an ambitious one. Called the Terrestrial Planet Finder, it would study the atmospheres of exoplanets and search for the chemical signatures of life: oxygen, water vapor, carbon dioxide. But congressional budget cuts have left the mission in limbo for now. It’s a project sorely in need of its own Bill Borucki.

“If there is no life in the entire galaxy, that would still be pretty profound for mankind,” Borucki says as we sit in his office. “So I think this is the most important problem we can attack. And I’m going to have to leave it to my grandkids.”

Despite Kepler’s jaw-dropping success, Borucki still frets about the mission, and one of my last questions touches a nerve. “By the end of Kepler’s mission, three years from now . . .” I begin to ask, but Borucki doesn’t let me finish.

“No!” he says, leaning forward over his desk, his soft voice rising in volume. “It’s going to go on at least six years. Don’t let people tell you that just because it’s funded for three and a half years it’s going to quit. It is not. We’re going to get the money and continue for six years. The program managers don’t believe that, but I believe it.”

Then, pausing after each word and rapping his knuckles on his desk for emphasis, he says, “You—can’t—shut—the mission—down. It’s working, and as we look longer and longer, we’ll find more and more small planets. Do you want to throw away an opportunity to find a huge number of planets in the galaxy because you didn’t come up with the money? It’s the only mission in the foreseeable future that can do this job and you’re gonna turn it off? Nobody’s gonna turn it off.”

KEPLER'S GREATEST HITS (SO FAR)

Kepler-10 b
About three-quarters of the planets discovered beyond our solar system are giant, gassy worlds. Several reside at the right distance from their stars to support liquid water, but balmy temperatures do not guarantee pleasurable conditions. Layer upon layer of gases can create pressure strong enough to crush a Brink’s truck like a paper cup, and the lack of terra firma leaves little place for life to take hold. In January Kepler astronomers announced the discovery of the first definitively rocky planet outside our solar system, Kepler-10 b. At 1.4 times the diameter of Earth, it also happens to be the smallest exoplanet yet measured. Unfortunately, rocky does not ensure habitable either. Daytime temperatures on Kepler-10 b approach 3000 degrees Fahrenheit. The good news is that Kepler’s latest results include 117 candidates at or below the size of Kepler-10 b and 23 smaller than Earth, strongly suggesting that the planet-hunting probe should soon find small, rocky exoplanets in kinder climates.

NN Ser (ab) c & d
About half the stars in the universe are gravitationally bound to a companion star. Coexisting with the heat and gravity of two suns is difficult for planets—especially for the pair discovered last year orbiting the binary star system NN Serpentis, also called NN Ser (ab). Astronomers believe that the exoplanets, called NN Ser (ab) c and d, may have survived a cataclysmic event several million years ago, when one of their host stars swelled to 200 times the diameter of the sun, temporarily enveloping the planets. Or perhaps the planets actually formed from material cast off during that expansion, a theory that would overturn our conventional understanding that planets and stars form together at the same time.

Kepler-11
By 2010 astronomers had discovered 54 stars hosting multiple planets, yet none of these planetary systems much resembled our own. In February researchers announced the discovery of a planetary system that looks slightly more familiar, Kepler-11. Six planets orbit a star roughly the size of the sun, and like our solar system, the outer planets are gas giants while the inner ones seem to be denser. But there’s a surprising twist: Five of the six planets are packed into orbits smaller than that of Mercury, their paths almost perfectly aligned in the same plane. Astronomers are at loss to explain how all the planets managed this configuration without crashing into each other.

KOI 326.01
Of the 1,235 planet candidates announced in February, KOI 326.01 truly stood out. Seemingly Earth-size and just a little hotter (140°F), it appeared to be the most Earth-like planet ever discovered—until a month later, when scientists made a less exciting discovery: an error in their data. Punching in the correct brightness of the planet’s star revealed a planet bigger and hotter than Earth. As disappointing as the news was, it was par for the course; picking out the tiny signatures of planets within trillions of bytes of data is notoriously difficult. In fact, about 10 percent of all “Kepler Objects of Interest” (KOIs) will end up as false positives, mission scientists say, while others will suffer demotions like that of KOI 326.01 based on follow-up observations. On the upside, Kepler researchers have discovered four other potential Earth-size planets with palatable temperatures.

GJ 1214 b
GJ 1214 b is nearly three times as wide and some 300ºF hotter than Earth, but it may nonetheless have an abundance of life’s most prized commodity: liquid water. Telescopic observations of the planet as it passed in front of its star indicated that GJ 1214 b may be covered with vast, hot oceans, which would remain liquid because of the extreme pressure exerted by the overlying atmosphere. GJ 1214 b also holds the title for orbiting closest to its star—a mere 1.3 million miles away, 1/70 the distance between Earth and the sun. Even though the star GJ 1214 is a puny red dwarf, it would still look 17 times larger from GJ 1214 b than the sun does in our sky.

Gas giant HD 80606 b is prone to wild weather. Winds there can reach up to 3 miles per second.

NASA/JPL-Caltech/G. Laughlin

HD 80606 b
In our solar system, every planet except Mercury revolves around the sun in a nearly perfect circular orbit. In fact, theorists long believed that planetary orbits had to be circular. Nature apparently believes otherwise. Take the unusual planet HD 80606 b, a gas giant in the constellation Ursa Major discovered in 2001. Its elongated path around its parent star is more reminiscent of the orbit of a comet than that of a planet the size of Jupiter. HD 80606 b swings within 3 million miles of its star at its closest approach and reaches its most distant point, 81 million miles away, just 56 Earth-days later. In 2009 astronomers reported that temperatures on the planet jumped from 1000°F to 2200°F in six hours.
—Andrew Grant

BANG ZOOM—TO THE STARS

Theoretical physicist Richard Obousy can easily imagine how the discovery might unfold: A successor mission to Kepler locates a planet that resembles our own in size and temperature. Further study shows intriguing chemical similarities as well—oxygen, water vapor, and methane, possible signatures of habitability, are all present. The next impulse would be to send a probe there to take a look. Does this Earth twin have oceans and continents? Signs of life? Signs of civilization?

Unfortunately, sending a spacecraft to even the closest star (Alpha Centauri, 4.3 light-years away) would take 70,000 years at the speed of today’s chemical rockets. Crossing the vast gulf of interstellar space within a human life­span will require a whole new wave of research. Obousy hopes to start that wave with Project Icarus, a collaboration between several dozen scattered scientists who are brainstorming concepts for the spacecraft of the future.

One possibility is a rocket powered by nuclear fusion, essentially a sequence of hydrogen bombs. The ship would be constructed in Earth orbit and then sent to Uranus to mine helium-3 fuel from the planet’s atmosphere. “That's really pushing technology to the edge,” says Marc Millis, head of the Tau Zero Foundation. He should know: Millis headed NASA’s Breakthrough Propulsion Physics project, which folded in 2002 without achieving the kind of radical advances they aspired to.

“If you look at the things within known physics that could actually take us to the stars, there are few options,” says Les Johnson, deputy manager of NASA’s Advanced Concepts Office. There are ultrathin solar sails pushed by the sun; Japan and the United States have already launched simple versions of this technology. The Ikaros sail, which took flight in May of last year, is only 14 square meters, however. Interstellar journeys would require a sail as big as Texas and orbiting lasers to provide a light boost; even then, the trip would take centuries, Johnson says.

There are also schemes to power electrical thrusters with nuclear fission—which, unlike fusion, is something we know how to do today. “Those designs are in NASA’s hands,” yellowing in a drawer somewhere, Millis says. The neglect stems not only from technical challenges, he explains, but also from political reluctance to put nuclear reactors in space.

And then there is the best kind of rocket physicists can imagine: engines that smash together matter and antimatter to produce pure energy. “The energy density is one to two orders of magnitude better than nuclear,” Millis says. But antimatter is no free lunch: You have to put in as much energy to make it in the first place as you get back when you annihilate it. Just synthesizing the requisite amount of antimatter (never mind figuring out how to use it in a rocket engine) lies far beyond humankind’s current capabilities.

No matter what the approach, going fast and far will require heaps of cash. Right now the money is not there; Tau Zero and Icarus are essentially self-funded projects run by hard-core enthusiasts. To make real progress on interstellar travel, NASA and the other space agencies will need a galvanizing spark. Like finding that second Earth.—Andrew Moseman